1. Field of The Invention
The present invention relates generally to a process for forming contact material. Specifically, the present invention relates to a process for forming contact material which may be used as an electrode of a vacuum interrupter.
2. Description of The Background Art
Commonly, as contact material which forms an electrode, higher current breaking ability is required when that is utilized for a vacuum interrupter.
Copper-Chromium (Cu-Cr) alloy is well known as contact material having good current breaking ability. Conventionally, Cu-Cr alloy is formed by powder metallurgy techniques, i.e., copper (Cu) powder prepared by electrolytic methods and chromium (Cr) powder prepared by milling are mixed then compacted under pressure. The compacted powder is sintered to obtain desired Cu-Cr alloy. In order to obtain a suitable electrode material indicating desired electric characteristics, homogeneous distribution of Cr into a Cu matrix is necessary. Further to say, the finer diameter of Cr particle, the better for the material.
However, particle distribution in materials of Cr prepared mechanically by milling methods becomes widely dispersed. Additionally, homogeneous fineness of Cr particle cannot be established easily because mean diameter of Cr particles becomes about 40 .mu.m. Therefore, weight variation occurs due to differing particle sized, differing specific gravity and differing distribution of particles, and such Cr particles cannot be homogeneously mixed with Cu powder. Therefore, after sintering, Cr particles cannot also be dispersed finely and homogeneously in the Cu matrix of a compacted article. Thus, electric characteristics of the article become degraded than those expected.
Commonly, Cu-Cr alloy is composed of a Cu matrix and Cr particles distributed therein. In order to obtain a desired electrode material having desired electric characteristics, Cr particle size must be decreased as fine as possible, and homogeneous distribution of such fine particles of Cr in the Cu matrix must be established.
Further milling of Cr particle using mechanical techniques is available to obtain fine particle size, but the surface of Cr particle is susceptible to the effects of oxygen in a course of mechanical processes. Therefore, oxidation of the Cr particle surfaces occurs in the process of milling and during storage. Sinterability of the mixed powder becomes reduced with increase of oxygen contained in Cr particle.
Classification of Cr particles using sieving means and selecting Cr particles only having fine particle diameter are effective for homogeneous distribution of fine particle, however, it causes severe degradation of yield and raises production cost.
Infiltration of Cr particle into voids generated in a compacted article of Cu particle, or infiltration of Cu particle into voids generated in a compacted article of a mixture of Cu and Cr after sintering at low temperature have been utilized to obtain desired characteristics. However, infiltratability of Cr particle becomes degraded because Cr at which surface is oxidized is difficult to wet. Generally, Cr particle tends to be easily oxidized, therefore, quality control of Cr particle is very difficult.
Casting methods for forming Cu-Cr alloy cannot be adopted, as the slow cooling speed of alloy solidification allows the size of Cr particles in the Cu matrix to be increased. Therefore, uniform distribution of fine Cr particles cannot be accomplished easily. Further to say, segregation is apt to occur during solidification. This causes quality of the article obtained from Cu-Cr alloy to be maldistributed.
Recently, atomization technique has been utilized for disintegrating a mixture of alloy elements into fine alloyed powder in place of using a mechanical milling technique.
However, in the process of atomization, oxygen content of Cr particle of material tends to be increased by certain amounts of impurities included therein. This increases oxygen content in an electrode obtained then degrades current breaking ability thereof. Additionally, Cr particle becomes difficult to melt because oxidized film is generated on the surface of the particle. Therefore, Cr particle becomes difficult to atomize from a nozzle. In order to sufficiently melt the particle, temperature of Cu-Cr molten alloy must be raised. However, because common temperature of producing the molten alloy is relatively high, i.e., 1600.degree. to 1700.degree. C., heat-stability of heated members, such as a heater, a heat insulator, and a crucible must be required to raise the temperature more than that of the aforementioned. This increases manufacturing cost.